Anode for chromium plating



Patented July 9, 1929.

UNITED STATES PATENT. OFFICE.

FREDERICK M. BECKET, OF NEW YORK, N. Y.

nonn non CHROMIUM IfLATING.

N Drawing.

purity which was commercially available,

hereafter for brevity designated pure chromium.

Anodes of pure cast chromium present a number of undesirable properties. Pure chromium is quite viscous evenwhen heated to a temperature well above its melting point, so that anodes can be cast from it only in the simplest shapes. Such anodes are phrous in varying degree, so that their rate of solution in the chromic acid electrolyte is irregular and impossible to predetermine.

Furthermore, the crystal structure is such to affect most unfavorably the manner in which the anodes dissolve: instead of dissolving smoothly and regularly they tend to disintegrate, pieces of various sizes spalling or sloughing oi? and collecting at the bottom of the electrolytic cell, this involving not only an increase in anode consumption but interfering with the operation of the cell.

These and perhaps other deficiencies of cast chromium anodes have been heretofore recognized, and have been summarized some what inaccurately by characterizing such anodes as brittle. It has been proposed to lessen this brittleness by addition of lead, cobalt, iron or nickel; and while some of these additions in some proportions may diminish the so-called brittleness of the anode, they have the disadvantage that eventually they eitherfoul the bath or have an undesirable effect upon the quality of the deposit.

According to the presentinvention the defects mentioned above are overcome, and further advantages are secured, by alloying the chromium with certain proportions of silicon, the amount of silicon varying with the objective to be attained, as explained below.

Silicon performs three distinct functions: 1. In the smaller percentages it increases the fluidity of the molten metal, facilitates the casting, and improves the grain of the cast article. These effects are already noticeable with silicon additions as low as 0.25 percent; but in. case these are the chief ob- A ppl i cation filed 0ctober14, 1926. Serial No. 141,678.

'jectives I prefer to introduce one to two percent of silicon, as already stated. I

2. 'It is characteristic of metallic chromium anodes, used in a chromic acidelectrolyte, that they tend to dissolve more rapidly than corresponds to the deposition of metal at the cathode, with the result that the concentration of the bath tends to increase progressively, and corresponding irregularities are introduced into the operation. This diificulty has been overco'me'in-the past by the simultaneous employment of soluble (chromium) and insoluble (lead) anodes, as disclosed in U. S. Patent 1,544,451 to Hambuechen. I have discovered that silicon, alloyed with the chromium in sufficient proportions, reduces the solubility of the chromium in the electrolyte, such as aqueous solutions of chromic acid. The solubility of the chromium diminishes progressively as the percentage of silicon is increased, until at about 40 percent silicon and upward, the balance almost all chromium, the alloy functions practically like an insoluble anode. Accordingly it is possible by properly adjustingthe silicon content so to control the rate of solution of chromium as to obtain the desired balance between the amount of metal dissolving at the anode and the amount deposited at the cathode, whereby the electrolyte remains within the limits of concen- .that the optium percentages of silicon lie between 10 and 35 percent; and in most cases betweenQO and 30 percen I 3. Silicon in sufliciently high proportion is capable of practically excluding carbon from the chromium-silicon alloy. For examplea chromium-silicon alloy containing about 40 percent of silicon, or upward, prepared by carbon reduction in an electric furnace of a charge of chromic oXid and silica, will contain only a fraction of one percent ofcarbon: and this practically carhon-free chromium-silicon alloy may be refurnaced with chromium oxid to oxidize out any desired proportion of the silicon. In

this way I am enabled to prepare alloys containing not more than 0.5 percent of carbon, the balance substantially all chromium and silicon, the silcon in any desired proi portion. Such alloys are regarded, for the purposes of this invention, as substantially carbon-free.

Having regard to 'allof the above-described 'functions of silicon', it will be understood that my present invention comprises an anode of cast chromium metal containing silicon n proportion between about 0.25 per- 1 cent and 35.0 percent, according to the par- 'ticular objectives to be attained, the alloy being substantially carbon-free or containing carbon not in excess of 0.5 percent. Cast chromium anodes containing carbon 0.5 to

ture.

' FREDERICK M. BECKET. 

